Reactions and reactivity of acyloxycarbenes

Article abstract of DOI:10.1021/ja9626244

Phenylacetoxycarbene, phenyl(pivaloyloxy)carbene, and phenyl(benzoyloxy)carbene, photolytically generated from diazirine precursors in pentane at 25°C, efficiently rearranged by 1,2-acyl migrations to give high yields of the appropriate 1,2-diketones. The kinetics of these rearrangements were determined by laser flash photolysis. Substituent effects on the acyl migrations and ab initio electronic structure calculations on ground state carbenes and transition states were employed to analyze the rearrangement mechanism. Additions of phenylacetoxycarbene to alkenes proceeded in good yields, in lieu of the 1,2-acyl shift; absolute rate constants were obtained for these reactions of the ambiphilic carbene. (Phenoxymethyl)acetoxycarbene gave only a 1,2-H shift; the potentially competitive 1,2-acetyl migration was suppressed.

Full text of DOI:10.1021/ja9626244

12588
J. Am. Chem. Soc. 1996, 118, 12588-12597
Reactions and Reactivity of Acyloxycarbenes
Robert A. Moss,* Song Xue, Weiguo Liu, and Karsten Krogh-Jespersen*
Contribution from the Department of Chemistry, Rutgers, The State UniVersity of New Jersey,
New Brunswick, New Jersey 08903
ReceiVed July 29, 1996X
Abstract: Phenylacetoxycarbene, phenyl(pivaloyloxy)carbene, and phenyl(benzoyloxy)carbene, photolytically gener-
ated from diazirine precursors in pentane at 25 °C, efficiently rearranged by 1,2-acyl migrations to give high yields
of the appropriate 1,2-diketones. The kinetics of these rearrangements were determined by laser flash photolysis.
Substituent effects on the acyl migrations and ab initio electronic structure calculations on ground state carbenes and
transition states were employed to analyze the rearrangement mechanism. Additions of phenylacetoxycarbene to
alkenes proceeded in good yields, in lieu of the 1,2-acyl shift; absolute rate constants were obtained for these reactions
of the ambiphilic carbene. (Phenoxymethyl)acetoxycarbene gave only a 1,2-H shift; the potentially competitive
1,2-acetyl migration was suppressed.
Oxa substituents profoundly affect the stability and reactivity
(trifluoroethoxy)carbene 2 than does the analogous form 1b to
methoxycarbene 1.
of carbenes.1,2 For example, the ground state of methylene is
a triplet,3 whereas that of dimethoxycarbene is a singlet,
calculated to lie 76 kcal/mol below the triplet.4 Moreover,
dimethoxycarbene2,4 and such monooxacarbenes as methyl-
methoxycarbene5 or phenylmethoxycarbene6 are strongly nu-
cleophilic; the electrophilic character apparent in the reactions
of methylene and (e.g.) the halocarbenes is suppressed.7,8
In theoretical terms, a strongly electron-donating substituent
like MeO (σ+R ) -0.66)9 raises both the LUMO and HOMO
energies of a singlet carbene, making the vacant LUMO less
accessible and the electrons of the HOMO more accessible, thus
In this light, the acetoxy substituent assumes importance
because it is a still weaker electron donor (σ+R ) -0.26)13 than
trifluoroethoxy, as suggested by resonance hybrid 3, where the
oxa lone pair can be delocalized over the carbonyl group (3c)
as well as the carbene center (3b). Indeed, on the basis of
substituent constants alone, acetoxycarbenes should be closer
in reactivity to the corresponding chlorocarbenes [σ+R (Cl) )
-0.21]9 than to the methoxycarbenes.14
accentuating the nucleophilic properties of the carbene.1,4,7
A
simple resonance representation of a methoxycarbene (1) makes
the same point through the contribution of form 1b.8
It should be possible to modulate the properties of oxacar-
benes by “fine-tuning” the donor potential of the oxa substituent.
For example, due to its inductively withdrawing trifluoromethyl
moiety, the trifluoroethoxy substituent is a weaker resonance
donor (σ+R ) -0.56),9 so that (trifluoroethoxy)carbenes are
somewhat more reactive and less nucleophilic than the corre-
sponding methoxycarbenes.10-12 In resonance terms, form 2b
makes a smaller contribution to the electronic distribution of
Acetoxycarbenes or, more generally, acyloxycarbenes are
relatively unexplored species. The cyclic example 4 was
generated in solution by photolysis of benzocyclobutene-3,4-
dione15 or in the gas phase by pyrolysis (560 °C) of benzoate
5.16 Flash vacuum pyrolysis of 5-acyloxy-4,6-dioxo-1,3-diox-
X Abstract published in AdVance ACS Abstracts, December 1, 1996.
(1) Rondan, N. G.; Houk, K. N.; Moss, R. A. J. Am. Chem. Soc. 1980,
102, 1770.
(2) Du, X. M.; Fan, H.; Goodman, J. L.; Kesselmayer, M. A.; Krogh-
Jespersen, K.; LaVilla, J. A.; Moss, R. A.; Shen, S.; Sheridan, R. S. J. Am.
Chem. Soc. 1990, 112, 1920.
(3) Mueller, P. H.; Rondan, N. G.; Houk, K. N.; Harrison, J. F.; Hooper,
D.; Willen, B. H.; Liebman, J. F. J. Am. Chem. Soc. 1981, 103, 5049.
(4) Moss, R. A.; Wlostowski, M.; Shen, S.; Krogh-Jespersen, K.; Matro,
A. J. Am. Chem. Soc. 1988, 110, 4443.
(5) Sheridan, R. S.; Moss, R. A.; Wilk, B. K.; Shen, S.; Wlostowski,
M.; Kesselmayer, M. A.; Subramanian, R.; Kmiecik-Lawrynowicz, G.;
Krogh-Jespersen, K. J. Am. Chem. Soc. 1988, 110, 7563.
(6) Moss, R. A.; Shen, S.; Hadel, L. M.; Kmiecik-Lawrynowicz, G.;
Wlostowska, J.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1987, 109, 4341.
(7) Moss, R. A. Acc. Chem. Res. 1989, 22, 15.
anes gave acyloxycarbenes [eq 1; R ) Me, Ph], which
(11) Ge, C.-S.; Jefferson, E. A.; Moss, R. A. Tetrahedron Lett. 1993,
34, 7549.
(12) Moss, R. A.; Jang, E. G.; Ge, C.-S. Pol. J. Chem. 1994, 68, 2501.
(13) Charton, M. In The Chemistry of Double Bonded Functional Groups;
Patai, S., Ed.; Wiley: New York, 1989; Vol. 2, Part 1, pp 239-298.
(14) On the contribution of substituent resonance donation (as represented
by σ+R ) to carbenic reactivity, see: ref 7 and 8.
(15) Staab, H. A.; Ipaktschi, J. Chem. Ber. 1968, 101, 1457. Brown, R.
F. C.; Solly, R. K. Tetrahedron Lett. 1966, 169. For a laser flash photolysis
study of the reaction of 4 with pyridine, adamantane thione, and several
alcohols, see: Boate, D. R.; Johnston, L. J.; Kwong, P. C.; Lee-Ruff, E.;
Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 8858.
(8) Moss, R. A. Acc. Chem. Res. 1980, 13, 58.
(9) Calculated by Professor M. Charton: Charton, M. Prog. Phys. Org.
Chem. 1981, 13, 119.
(10) Moss, R. A.; Liu, W.; Ge, C.-S. J. Phys. Org. Chem. 1993, 6, 376.
S0002-7863(96)02624-8 CCC: $12.00 © 1996 American Chemical Society

Reactions and ReactiVity of Acyloxycarbenes
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12589
afforded diones by an apparent 1,2-acyl shift.16-18 Analogous
generation of methyl(benzoyloxy)carbene [eq 1; R ) Me, 560
°C] labeled with 18O at its carbonyl oxygen gave a benzoyl-
labeled dione product, excluding significant oxygen scrambling
during the lifetime of the carbene.18
presumably via a SRN1 mechanism of the type proposed by
Creary,25 in which the phenyldiazirinyl radical 11 is a presump-
tive intermediate.24,25
Mild thermolysis of the oxadiazolines 6 (R ) Me, Et) gave
methyl- and ethylacetoxycarbenes, as well as dicyclopropyl-
carbene.19 Among the products were the diones expected from
acetyl shifts of the acetoxycarbenes. Finally, treatment of (e.g.)
the chloromethylpivalate 7 with lithium tetramethylpiperidide
in refluxing alkene-ether afforded modest yields of cyclopro-
panes derived from pivaloylcarbene or pivaloylcarbenoid.20
In parallel applications of eq 2, reactions of 8 with TBA (tetra-
n-butylammonium) pivalate and of 3-(phenoxymethyl)-3-bro-
modiazirine26 with TBAA gave 3-phenyl-3-(trimethylacetoxy)-
diazirine (12, 60%) and 3-(phenoxymethyl)-3-acetoxydiazirine
(13, 42%),21 respectively.
The reaction of diazirine 8 with TBA benzoate in DMF did
not give 3-phenyl-3-(benzoyloxy)diazirine (14); benzonitrile
(presumably derived from the dimer of 11)27 was formed instead.
Apparently, benzoate anion was not reactive enough to capture
diazirinyl radical 11, but with both TBA acetate and benzoate
present, 8 gave 32% of acetoxydiazirine 9, 28% of (benzoyloxy)-
diazirine 14, and 5% of phenyldiazirine 10.24 A similar outcome
The foregoing methods for the generation of acyclic acyl-
oxycarbenes require high temperature, proceed in low yield, or
may not involve free carbenes. Moreover, they are not suitable
for the generation of acyloxycarbenes under kinetically acces-
sible conditions. In 1994, we communicated the preparation
of 3-phenyl-3-acetoxydiazirine and the generation, reactions, and
associated absolute kinetics of the phenylacetoxycarbene (Ph-
COAc) that was photolytically derived from this precursor.21
Now we present full details of the chemistry of PhCOAc and
several related acyloxycarbenes.
The defining intramolecular reaction of the acyloxycarbenes
is the 1,2-acyl shift that affords a 1,2-dione; see, for example,
eq 1.16-19 Here we will consider the absolute rate constants
and activation parameters that characterize this transformation,
its structural dependence, an appropriate mechanistic formula-
tion, and the efficiency of competitive intramolecular reactions
(e.g., the 1,2-H shift). We will also describe the intermolecular
reactivity and philicity toward alkenes of PhCOAc.
accompanied the reaction of 8 with KOAc and potassium
benzoate solubilized in DMF with 18-crown-6.24 The require-
ment for acetate ion in the formation of 14 is attributed to the
necessity of prior N-capture of radical 11 by acetate ion, with
the formation of N-acetoxyisodiazirine (15), from which 14
arises by SN2′ reaction with benzoate.25,28 (Competitive SN2′
reactions of 15 with acetate afford 9.)
Results
Similar “synergistic” diazirine exchange reactions of diazirine
8, with acetate and either p-methyl- or p-methoxybenzoate ions
as nucleophiles, gave 43% of 3-(p-methyl(benzoyloxy))-3-
phenyldiazirine (16) or 80% of 3-(p-methoxy(benzoyloxy))-3-
phenyldiazirine (17), accompanied by 25 or 7%, respectively,
of acetoxydiazirine 9. Unfortunately, electron-withdrawing
group substituted benzoates (e.g., p-chlorobenzoate) were unable
to compete with acetate ion for 15 (nor for 11), so that only 14,
16, and 17 were prepared in the (benzoyloxy)phenyldiazirine
series.
Diazirines. The halodiazirines that are readily obtained by
the hypohalite oxidation of amidines22 can often be converted
to other functionalized diazirines by a nucleophilic exchange
reaction.7,23 Indeed, stirring phenylbromodiazirine (8)22 with
excess tetra-n-butylammonium acetate (TBAA) under air in dry
DMF for 20 min at 25-30 °C afforded 45% of phenylacetoxy-
diazirine (9), as well as 15% of phenyldiazirine (10) (eq 2),21,24
(16) Brown, R. F. C.; Eastwood, F. W.; McMullen, G. L. J. Chem. Soc.,
Chem. Commun. 1975, 328.
(17) Brown, R. F. C.; Eastwood, F. W.; Lim, S. T.; McMullen, G. L.
Aust. J. Chem. 1976, 29, 1705.
(18) Brown, R. F. C.; Browne, N. R.; Eastwood, F. W. Aust. J. Chem.
1983, 36, 2355.
(19) Bekhazi, M.; Warkentin, J. J. Org. Chem. 1982, 47, 4870.
(20) Olofson, R. A.; Lotts, K. D.; Barber, G. N. Tetrahedron Lett. 1976,
3381.
(21) Moss, R. A.; Xue, S.; Liu, W. J. Am. Chem. Soc. 1994, 116, 1583.
(22) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(23) For a review, see: Moss, R. A. In Chemistry of Diazirines; Liu, M.
T. H., Ed.; CRC Press: Boca Raton, Florida, 1987; Vol. I, pp 99-109.
(24) Moss, R. A.; Xue, S.; Liu, W. J. Am. Chem. Soc. 1994, 116, 10821.
Diazirine yields are based on consumed bromodiazirine.
(25) Creary, X. Acc. Chem. Res. 1992, 25, 31. Creary, X.; Sky, A. F.;
Phillips, G. J. Org. Chem. 1990, 55, 2005. Creary, X.; Sky, A. F.; Phillips,
G.; Alonso, D. E. J. Am. Chem. Soc. 1993, 115, 7584. Creary, X. J. Org.
Chem. 1993, 58, 7700.
(26) Moss, R. A.; Ho, G.-J.; Liu, W. J. Am. Chem. Soc. 1992, 114, 959.
(27) Maeda, Y.; Ingold, K. U. J. Am. Chem. Soc. 1979, 101, 837.
(28) Daily, W. P., III Tetrahedron Lett. 1987, 28, 5801. Bainbridge, K.
E.; Daily, W. P., III Ibid. 1989, 30, 4901. Creary, X.; Sky, A. F. J. Am.
Chem. Soc. 1990, 112, 368.

12590 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Moss et al.
We also prepared the p-chloro and p-trifluoromethyl ana-
logues of 9, diazirines 18 (69%) and 19 (54%), by reactions of
the appropriate arylbromodiazirine29 with TBAA in DMF.
Because these acetate-diazirine exchange reactions are initiated
by electron transfer to the initial arylbromodiazirine,24,25 electron-
withdrawing groups on the aryl moiety accelerate the reaction.24
Conversely, electron-donating substituents inhibit the process,
and we could not obtain the p-methoxy analogue of 18 using
(p-methoxyphenyl)bromodiazirine as the substrate. Thus, 9, 18,
and 19 are the available representatives in the arylacetoxydi-
azirine series.
All of the diazirines were purified by silica gel chromatog-
raphy and characterized by UV and NMR spectroscopy; details
appear in the Experimental Section.
Carbene Kinetics; Ylide Formation. Laser flash photolysis
(LFP)6 at 351 nm of diazirine 9 (A366 ) 0.4) at 25 °C in pentane
revealed a weak absorbance at ∼310 nm for PhCOAc that was
not suitable for precise kinetic study. We therefore made use
of the pyridine ylide methodology:30 LFP of 9 in pentane
containing 0.0122-0.0589 M pyridine gave PhCOAc (20) and
then ylide 21 (eq 3).21,30
PhCOAc in the absence of pyridine.30 The yield of dione
exceeded 90%, and no azine formation was observed (from
reaction of 9 and PhCOAc), so that we can equate the
extrapolated rate constant kre ) (1.3 ( 0.2) × 105 s-1 (r >
0.999) with the rate constant for the rearrangement of carbene
20 to dione 22.32
Similarly, we examined analogous reactions of phenyl-
(pivaloyloxy)carbene (23) and phenyl(benzoyloxy)carbene (24)
with pyridine. Photolysis for 1-1.5 h of diazirines 12 (A366
)
0.4) or 14 at 25 °C (A368 ) 0.4) with λ > 320 nm in pentane
solutions caused the disappearance of the diazirines and the
formation of diones 25 or 26, respectively, in >90% conversion.
The products were identified by NMR spectroscopy and either
elemental analysis (25) or comparison with an authentic sample
(26). Thermolysis of 14 in pentane (60 °C, 30 h, sealed tube)
afforded >98% of benzil.
The kinetics of these rearrangements were determined by LFP
using the pyridine ylide method as outlined above. For diazirine
12 and carbene 23, [pyridine] was varied from 1.22 to 5.89 ×
10-2 M in isooctane at 20 °C, affording the appropriate ylide
which was monitored at 470 nm. From the correlation (r >
0.999), the slope kylide ) (7.4 ( 0.1) × 107 M-1 s-1 and the
intercept at [pyridine] ) 0 kre ) (4.9 ( 0.5) × 105 s-1. The
latter can be taken as the rate constant for the 1,2-pivaloyl shift.
For diazirine 14 and PhCOOCPh (24), [pyridine] was varied
from 0.61 to 3.01 × 10-2 M in isooctane at 20 °C, affording
the appropriate ylide upon LFP. The correlation of kobsd for
ylide formation vs [pyridine] (r > 0.999) gives kylide ) (2.1 (
0.1) × 108 M-1 s-1 and kre ) (6.7 ( 0.7) × 105 s-1, which we
take as representative of the 1,2-benzoyl shift of PhCOOCPh
to benzil.
The ylide featured a strong absorption, λmax 470-480 nm,
similar to the signal observed for the analogous ylide of
phenylchlorocarbene at 480 nm.30,31 From the slope of the
observed linear dependence of the pseudo-first-order rate
constants for the formation of 21 vs the concentration of
pyridine, we obtain kylide ) (3.5 ( 0.1) × 107 M-1 s-1 for the
second order reaction between PhCOAc and pyridine.21,31 As
we indicated previously, this is a “middling” value for carbene-
pyridine ylide formation, indicative of an oxacarbene of
modified reactivity in accord with structure 3.21,31 We return
to this point below.
Kinetics of Dione Formation. Photolysis of a pentane
solution of diazirine 9 (A366 ) 0.4) for 1 h at λ >320 nm
destroyed the diazirine and afforded >90% of dione 22,
identified by NMR and GC-MS comparisons to an authentic
sample.21 A similar yield of 22 was obtained from thermolysis
of 9 in pentane at 60 °C for 30 h (sealed tube).
Arrhenius Study of Phenyl(benzoyloxy)carbene. In pre-
cisely the same manner, kre was determined for the rearrange-
ment of carbene 24 at 253, 268, 283, and 303 K. Reaction
temperatures were measured with an indwelling thermocouple
connected to a digital voltmeter. The correlations of kobsd for
ylide formation vs [pyridine] were all good (r > 0.998) and
afforded kylide values ranging from 1.6 to 2.2 × 108 M-1 s-1
.
The rate constants for the 1,2-benzoyl shifts were 7.18 × 104
(253 K), 1.59 × 105 (263 K), 4.24 × 105 (283 K), 6.69 × 105
(293 K), and 1.14 × 106 s-1 (303 K). Errors of ∼15% are
estimated for these rate constants. The corresponding Arrhenius
correlation (r ) 0.999) afforded the parameters Ea ) 8.4 ( 0.2
kcal/mol and log A ) 12.1 ( 0.1 s-1 (corresponding to ∆S* )
-5.0 eu at 298 K). The “high” activation energy and small
negative entropy of activation are noteworthy and will be
discussed below.
Substituent Effects. In order to probe the electronic
character of representative acyl shift transition states, we
determined kre values (in isooctane) for several substituted
phenyl(benzoyloxy)carbenes and phenylacetoxycarbenes. From
diazirines 14, 16, and 17, using the pyridine ylide methodology,
we obtained kre for carbenes 27a-c, whereas, from diazirines
9, 18, and 19, we obtained kre for carbenes 28a-c.
Extrapolation of the correlation between the observed LFP
rate constant of ylide formation and [pyridine] to [pyridine] )
0 gives the aggregate rate constant for all processes that destroy
(29) Moss, R. A.; Terpinski, J.; Cox, D. P.; Denney, D. Z.; Krogh-
Jespersen, K. J. Am. Chem. Soc. 1985, 107, 2743.
(30) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J.
Am. Chem. Soc. 1988, 110, 5595. Jackson, J. E.; Platz, M. S. In AdVances
in Carbene Chemistry; Brinker, U. H., Ed.; JAI Press: Greenwich, CT,
1994; Vol. 1, pp 89f.
(31) Ge, C.-S.; Jang, E. G.; Jefferson, E. A.; Liu, W.; Moss, R. A.;
Wlostowska, W.; Xue, S. J. Chem. Soc., Chem. Commun. 1994, 1479.
(32) For a review of the kinetics of 1,2-carbenic rearrangements, see:
Moss, R. A. In AdVances in Carbene Chemistry; Brinker, U. H., Ed.; JAI
Press: Greenwich, CT, 1994; Vol. 1, pp 59ff.

Reactions and ReactiVity of Acyloxycarbenes
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12591
Table 1. Rate Constants for Acyl Shiftsa
Table 2. Experimental Relative Reactivities of PhCOAc at 25 °Ca
b
c
b,c
carbene
substituent
σp
105 kre (s-1
)
case
olefin a
olefin b
ka/kb
27ad
27b
27c
H
0.00
-0.14
-0.28
0.00
0.24
0.53
6.69
9.24
12.4
1.33
2.44
2.56
1
2
3
4
CH2dCHCN
CH2dCHCN
CH2dCHCN
Me2CdCH2
trans-MeCHdCHMe
Me2CHdCH2
Me2CHdCHMe
Me2CHdCHMe
15.7 ( 0.7
2.44 ( 0.03
3.56 ( 0.19
1.44d
Me
MeO
H
Cl
CF3
28ae
28b
28c
a See text for details. b Where appropriate, relative reactivities are
based on composites of syn and anti isomeric cyclopropane adducts.
c Errors are average deviations from the means of two determinations.
d Single experiment.
a At 20 °C in isooctane; see text. b Reference 33. c Estimated error,
(15%. d Identical with 24. e Identical with 20.
Ylide signals were monitored at 470 nm at 20 °C. In each
case, preparative photolysis of the diazirine in pentane gave
g90% of the expected dione which was isolated and character-
ized (see Experimental Section). Each kinetic determination
employed six or more pyridine concentrations, and the resulting
kobsd vs [pyridine] correlations had r g 0.998. The kinetic data
appear in Table 1.
afforded cyclopropanes 31a-f, adducts of PhCOAc, in yields
of 60-90%; syn-anti-cyclopropane isomers were not separated.
In some cases, the cyclopropanes were further purified by
preparative GC. The adducts were characterized by NMR
spectroscopy, GC-MS, and either high-resolution MS or el-
emental analysis. Details appear in the Experimental Section.
The kre data for the substituted (benzoyloxy)carbenes 27a-c
correlate nicely with σp(X)33 affording a Hammett F ) -0.96
(r ) 0.99). Donating substituents on the migrant benzoyl groups
of 24 (27a) therefore mildly accelerate these benzoyl shifts. On
the other hand, the data show that electron-withdrawing groups
at the migration termini of the carbenes accelerate the acetyl
shifts of the substituted phenylacetoxycarbenes 28a-c.
A
satisfactory Hammett correlation is not observed, however, and
the CF3-substituted carbene 28c rearranges more slowly than
anticipated from its σ value, relative to carbenes 28a (20) and
28b. The substituent effects will be discussed below.
A Competitive 1,2-H Shift. Photolysis of diazirine 13 in
pentane solution (λ > 320 nm, A328 ) 1.0, 25 °C, 36 h) gave
only the 1,2-H shift product alkene cis-29, expected from
(phenoxymethyl)acetoxycarbene (30); there was no evidence for
a 1,2-acetyl shift.21 The structure of 29 was secured by NMR
Note that the high yields of olefin adducts and the absence
of dione 22 in these product mixtures signify that the dione is
a product of carbene 20; it is not formed by an acyl shift of
excited diazirine 9. The kinetics of the PhCOAc alkene addition
reactions were determined by a combination of absolute and
relative rate measurements. For acrylonitrile, chloroacrylonitrile,
and methyl acrylate, absolute rate constants were determined
by the pyridine ylide method:21,30 diazirine 9 (A366 ) 0.8-1.0)
was subjected to LFP, generating PhCOAc in pentane or
isooctane solutions containing a fixed concentration of pyridine
(0.0247 M) and variable concentrations of acrylonitrile (0.0234-
0.642 M), chloroacrylonitrile (0.0250-0.125 M), or methyl
acrylate (0.214-0.808 M). Correlations of the apparent rate
constants for the formation of ylide 21 as a function of alkene
concentration were linear (r > 0.998 for 5-7 points), with slopes
that were equivalent to kaddn for the carbene-alkene reactions.
We thus obtained kaddn (M-1 s-1) ) (1.20 ( 0.09) × 106 (methyl
acrylate), (3.54 ( 0.04) × 106 (acrylonitrile), and (5.1 ( 0.1)
× 107 (chloroacrylonitrile) for these PhCOAc additions.
Rate constants for trimethylethylene, isobutene, and trans-
butene were initially determined relative to acrylonitrile by
standard competitive methods34 and then converted to absolute
rate constants using the kabs value for acrylonitrile (determined
above).
Diazirine 9 was photolytically decomposed (λ > 320 nm, 25
°C) in binary mixtures of at least 10-fold excess of alkenes.
The product cyclopropanes (31a-c,e), which were stable to the
reaction conditions, were quantitated by capillary GC using a
flame ionization detector and an electronic integrator. The
relative reactivity of alkene a vs alkene b was calculated from
(ka/kb) ) (Pa/Pb) (Ob/Oa), where Pi is the mole fraction of
product cyclopropane and Oi is the initial alkene mole fraction.34
The experimentally determined relative reactivities appear in
Table 2.
spectroscopy, GC-MS, and elemental analysis. Its cis config-
uration follows from the small (3.7 Hz) vinyl H-coupling
constant; a similar preference for Z-alkene products from the
1,2-H shifts of PhOCH2CX has been observed with X ) Cl or
F.26
LFP of diazirine 13 (A328 ) 0.8) in pentane containing (1.87-
6.24) × 10-3 M pyridine gave a strong ylide signal at 470 nm.
From the slope and intercept of the correlation (r ) 0.997, 8
points) between the rate constants for ylide growth and
[pyridine], we obtained kylide ) (1.03 ( 0.06) × 109 M-1 s-1
and kH ) (4.1 ( 0.3) × 106 s-1 for the 1,2-H shift of carbene
30 to alkene 29.21
When diazirine 13 was photolyzed in acrylonitrile (A328
)
1.0, λ > 320 nm, 2 h), GC-MS showed that <1% of alkene 29
had been formed; >99% of the product mixture consisted of
the isomeric 1-acetoxy-1-(phenoxymethyl)-2-cyanocyclopro-
panes expected from the addition of carbene 30 to acrylonitrile.
The near absence of H-shift product 29, under conditions where
30 is scavenged by an alkene, indicates that 29 stems almost
exclusively from carbene 30 and not from H-shift in an excited
state of the diazirine.32
Intermolecular Additions. Photolysis of diazirine 9 (λ >
320 nm, 2 h, 25 °C) in trimethylethylene, isobutene, trans-
butene, methyl acrylate, acrylonitrile, or R-chloroacrylonitrile
(33) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley: New York,
1992; p 280.
(34) For a review, see: Moss, R. A. In Carbenes; Jones, M., Jr., Moss,
R. A., Eds.; Wiley: New York, 1973; Vol. 1, pp 153ff.

12592 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Moss et al.
Reproducibilities are good; the highest uncertainty, associated
with trimethylethylene, is <5.5%. A satisfactory cross check
experiment34 was performed: from cases 2 and 3 of Table 2,
we calculate krel (Me2CdCH2/Me2CdCHMe) ) 1.46, and the
experimentally determined value (case 4) is 1.44.
Recalling that kabs for addition of PhCOAc to acrylonitrile is
3.54 × 106 M-1 s-1 (see above), the krel data of Table 2 affords
kabs for PhCOAc additions to trimethylethylene, isobutene, and
trans-butene of 0.99 × 106, 1.45 × 106, and 0.23 × 106 M-1
s-1, respectively. We estimate errors of ∼10% in these values.
These data will be considered further below.
Ab Initio Electronic Structure Calculations. A computa-
tional study of acyloxycarbenes was undertaken using standard
ab initio electronic structure methods35 implemented in the
Gaussian 94 series of programs.36 The calculations were made
at the levels of Hartree-Fock (HF) and second order Møller-
Plesset perturbation theory (MP2)37 with STO-3G, 4-31G, and
6-31G* basis sets.38,39
transition state separating the two conformers is located at
dihedral angle C1O1C2O2 ≈ 75°, and the barrier height is 3.0
kcal/mol above 28a-E (but only 0.1 kcal/mol above 28a-Z) at
the MP2/6-31G*//HF/6-31G* level. At thermal equilibrium at
ambient temperatures, the population ratio of Z/E conformers
should thus not only be less than 1% but also, considering the
small barrier separating the Z from the E minimum, it appears
possible that 28a-Z might not survive as a minimum at high
levels of theory.42
Replacing Me by Ph on the acetoxy carbon (28a f 27a)
leads, as expected, to only very small structural changes in the
C,O backbone, that is, bond lengths and bond angles in 27a-Z
are superimposable onto those of 28a-Z to better than 0.01 Å
and 1° accuracy. Both phenyl groups are oriented for optimal
conjugation with the skeletal framework in 27a-Z which, in
addition, shows a slightly larger rotation around the O1-C2 bond
( C1O1C2O2 ) 34.4°) than does 28a-Z (21.5°). There is no
minimum corresponding to a 27a-E conformer, however, only
a transition state for internal rotation around the O1-C2 bond
could be located in the dihedral angle region expected for an E
conformer ( C1O1C2O2 ≈ 180°). The energy difference
between 27a-Z and this transition state for internal rotation is
3.3 kcal/mol (MP2/6-31G*//HF/6-31G*). The phenyl group on
C2 must rotate away from coplanarity with the carbonyl and
the skeletal C,O atoms in this dihedral angle region and the
loss of electronic stabilization, combined with the steric bulk
of the phenyl group, presumably are the factors responsible for
the absence of an E conformer for 27a.
Introduction of the various substituents on the phenyls of the
parent acyloxycarbenes (27a f 27b,c; 28a f 28b,c) affects
only the local phenyl geometries and does not change the ground
state E vs Z conformational preference exhibited by the parent
species. Thus, for 28b and 28c the Z conformers are preferred
by 2.7 and 2.5 kcal/mol, respectively.
A large vertical singlet-triplet splitting was computed in 28a
(∆EST ≈ 100-105 kcal/mol), and the carbenes considered here
clearly all possess singlet ground states and react accordingly.
The delocalization of O1 and Ph π electrons into the formally
empty carbenic p orbital leads to a destabilized, high lying
carbene LUMO, whereas the σ electron-withdrawing character
of O1 stabilizes the carbene HOMO, viz., ꢀHOMO(σ) ) -10.13
(9.82) eV and ꢀLUMO(p) ) 1.61(1.77) eV in 28a-E(Z), -9.90
and 1.62 eV, respectively, in 27a-Z (HF/4-31G//STO-3G orbital
energies quoted to conform with previous usage).1,7 On the
basis of these orbital energies, we predict that 27a and 28a (and
the related carbenes) should behave as ambiphiles in reactions
Minima on the ground state potential energy surfaces of 22,
27a-c, and 28a-c and the transition states for 1,2-acyl shifts
(27aTS-cTS, 28aTS-cTS) or internal rotation were located40
at the HF level with the 6-31G* split-valence plus polarization
function basis set (HF/6-31G*//6-31G*). Harmonic normal
mode analysis on the parent systems (22; 27a and 27aTS; 28a
and 28aTS) characterized the located stationary points as
minima or transition states and provided thermodynamic data
for zero-point vibrational energy and finite temperature correc-
tions.35 Improved relative energies were obtained from single
point MP2/6-31G* calculations at the optimized HF geometries
(MP2/6-31G*//HF/6-31G*). Molecular charge distributions
were partitioned into orbital and atomic charges using the
techniques developed by Weinhold et al. (NBO analysis).41
Ground State Geometries of Acyloxycarbenes. Most
structural features observed in the ground state of PhCOAc (28a)
are representative of all carbenes considered in this computa-
tional study. Minima are found for two orientations of the
COOCMe unit, corresponding to dihedral angles C1O1C2O2 ∼
0° (28a-Z) or ∼180° (28a-E), as illustrated by the HF/6-31G*-
optimized structures shown below. Conformer 28a-Z optimizes
to a fully planar structure (ignoring the two H’s on C3). In
28a-E, however, a small rotation occurs around the O1-C2 bond
( C1O1C2O2 ) 21.5°), and the phenyl group is also rotated a
few degrees away from coplanarity with the skeleton ( C4-
(phenyl)C1O1C2 ) 176.2°).
Conformer 28a-E is lower in energy than conformer 28a-Z
by 2.9 kcal/mol at the MP2/6-31G*//HF/6-31G* level. The
(35) Hehre, W. J.; Radom, L.; Pople, J. A.; Schleyer, P. v. R. Ab Initio
Molecular Orbital Theory; Wiley-Interscience: New York, 1986.
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. S.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN94, Revision B.1;
Gaussian, Inc.: Pittsburgh, PA, 1995.
(37) (a) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618. (b) Krishnan,
R.; Frisch, M. J.; Pople, J. A. J. Chem. Phys. 1980, 72, 4244.
(38) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724. Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(39) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650.
(40) Schlegel, H. B. In New Theoretical Concepts for Understanding
Organic Reactions; Bertran J., Ed.; Kluwer Academic: The Netherlands,
1989; p 33.
(41) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899.
(42) Structures 28a-c and 28aTS-cTS were reoptimized with electron
correlation included at the MP2/6-31G* level. The MP2/6-31G*//6-31G*-
optimized structures are not dramatically different from those obtained at
the HF/6-31G*//6-31G* level, in particular conformer 28a-Z does persist
as minimum at this computational level.

Reactions and ReactiVity of Acyloxycarbenes
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12593
with alkenes,7,8 a prediction verified for 28a by the experimental
data (see below).
Discussion
Reactivity. Alkoxycarbenes such as RCOMe are strongly
stabilized (cf., 1) and therefore considerably less reactive and
more nucleophilic than (e.g.) halocarbenes.1-8 Moreover, the
electronic and stabilizing effects of an oxa substituent can be
modulated by “fine-tuning” the electronic character of the
substituent. The trifluoroethoxy group, for instance, is less
donating than methoxy, so that RCOCH2CF3 (cf., 2) is a more
reactive, less nucleophilic carbene than RCOCH3.10-12 Ad-
ditional weakening of electron donation, coupled with decreased
carbene stabilization and increased reactivity, is anticipated for
acyloxy-substituted carbenes (cf., 3),13 and our results support
this expectation.
Figure 1. (a) Acylanion-like pathway for the acyloxycarbene to dione
rearrangement; the “terminus” is the carbene LUMO. (b) Carbanion-
like attack on the carbonyl group as an alternative pathway for the
same rearrangement. The principal interaction is carbene-HOMO/
carbonyl-LUMO, and the carbonyl group acquires negative charge in
the transition state.
Thus, kylide for the reaction of PhCOAc with pyridine (eq 3)
is 3.5 × 107 M-1 s-1, representing rate constant increases of
19 and 292 relative to comparable reactions of PhCOCH2CF3
(1.8 × 106 M-1 s-1) and PhCOCH3 (1.2 × 105 M-1 s-1),
respectively.31 Note, however, that the reactivity of PhCOAc
toward pyridine remains 43 times lower than that of PhCCl (1.5
× 109 M-1 s-1), so that the deactivating properties of the
resonance-donating oxa substituent persist in the acetoxy
substituent, even though they are mitigated.
The reactivities of phenyl(pivaloyloxy)carbene (23) and
phenyl(benzoyloxy)carbene (24) are slightly greater than that
of PhCOAc, with kylide values of 7.4 × 107 and 2.1 × 108 M-1
s-1, respectively.
tion entropies this negative have been associated with H or even
C tunneling, an attribution supported by calculations and (in
the case of MeCCl) by a curved Arrhenius correlation of kH vs
1/T.46 In this light, the mildly negative ∆S* and linear Arrhenius
behavior of PhCOOCPh (at least to -20 °C) would suggest
that tunneling is unimportant in this 1,2-acyl shift near ambient
temperature. Additionally, as suggested by a referee, the
negative contributions to ∆S* arising from restrictions on
internal rotations that operate in 1,2-H or 1,2-C shifts would
be mitigated in 1,2-acyl migrations where the migration origin
is an oxygen atom, and rearrangement creates a carbonyl group
rather than an alkene.
Combining relative energies (MP2/6-31G*//HF/6-31G*) with
thermal and zero-point vibrational energy corrections (HF/6-
31G*//6-31G*), we obtain a calculated Ea value for the 1,2-
acyl shift of PhCOOCPh (27a-Z) of 5.1 kcal/mol and ∆S* )
-3.3 eu, in (fortuitously?) rather good agreement with the
experimental values determined above as 8.4 kcal/mol and -5.0
eu, respectively. Similarly, for the rearrangement of PhCOAc
(28a-E), we obtain Ea ) 7.0 kcal/mol and ∆S* ) -3.0 eu.
Viewed differently, the calculated ∆G* ) 6.1 kcal/mol for
PhCOOCPh, whereas the experimental data lead to ∆G* ≈ 9.9
kcal/mol; for PhCoAc, the calculated ∆G* ) 7.8 kcal/mol. Note
that even the small difference in acyl shift rate constants between
PhCOOCPh and PhCOAc (the former reacts ∼5 times faster
than the latter, Table 1) is qualitatively born out by the calculated
∆G* values (6.1 vs 7.8 kcal/mol). We note also that the 1,2-
acyl shift to form the dione is accompanied by a substantial
release of energy. For example, the reaction 28a-E f 22 is
computed to be exothermic by 54.5 kcal/mol (MP2/6-31G*//
HF/6-31G*).
The Transition State. A priori, we might imagine two
“extreme” transition states for the 1,2-acyl rearrangement of the
acyloxycarbenes. One pathway, Figure 1a, traverses an acyl
anion-like transition state, in which the C-O bond that breaks
is eclipsed with the vacant carbene p orbital or LUMO. In the
transition state, analogous to that of a 1,2-hydride migration,
the carbene LUMO is the migration terminus, the C-O
σ-bonding pair migrates with the acyl moiety, and the carbene
center should acquire negative charge. The second pathway,
Figure 1b, features a carbanion-like attack of the carbene σ pair
(HOMO) on the carbonyl LUMO (π*). In this case, the
carbonyl group would assume negative charge in the transition
state.
The signature intramolecular reaction of an acyloxycarbene
is its 1,2-acyl migration to afford a dione; by such rearrange-
ments, diones 22, 25, and 26 arise from carbenes 20, 23, and
24, respectively. The attendant rate constant (s-1) are 1.3 ×
105 (20), 4.9 × 105 (23), and 6.7 × 105 (24). Compared to
many 1,2-H shifts, these acyl migrations are relatively “slow”;32
the lifetime of PhCOAc in pentane, for example, is ∼8 µs. Note
that steric effects do not retard the rearrangement of the pivaloyl
group of 23, which occurs 3.8 times more rapidly than the acetyl
shift of PhCOAc. The rearrangement of 24 to benzil by a
benzoyl shift is even faster, 5.2 times more rapid than that of
PhCOAc. The acetyl shift of the latter is comparable in rate to
the 1,2-C shift ring expansion of cyclopropylchlorocarbene.32,43
The rearrangement of PhCOOCPh (24) to benzil was
subjected to an Arrhenius study that afforded Ea ) 8.43 kcal/
mol, log A ) 12.1 s-1, and ∆S* ) -5.0 eu. The activation
energy is the highest experimental value yet reported for a 1,2-
carbenic rearrangement of any type;32 1,2-carbon shifts of
cyclopropylfluorocarbene26 and cyclopropylchlorocarbene,43 for
example, occur with Ea ) 4.2 or 3.0 kcal/mol, respectively.44
Noteworthy is the near-neutral ∆S* (-5.0 eu) determined
for the PhCOOPh rearrangement. The mildly negative activa-
tion entropy is consistent with a moderate degree of ordering
in the transition state and stands in contrast to the strongly
negative ∆S* values reported for other carbenic rearrange-
ments.32 For example, these parameters for the 1,2-H shift of
methylchlorocarbene45 and the 1,2-C shift of cyclopropylchlo-
rocarbene43 are -16 and -20 to -24 eu, respectively. Activa-
(43) Moss, R. A.; Ho, G.-J.; Shen, S.; Krogh-Jesperson, K. J. Am. Chem.
Soc. 1990, 112, 1638. Ho, G.-J.; Krogh-Jespersen, K.; Moss, R. A.; Shen,
S.; Sheridan, R. S.; Subramanian, R. Ibid. 1989, 111, 6875.
(44) However, see: Liu, M.TH.; Bonneau, R. J. Am. Chem. Soc. 1989,
93, 7298. The cyclopropylchlorocarbene 1,2-C shift is assigned Ea ) 7.4
kcal/mol.
(45) LaVilla, J. A.; Goodman, J. L. J. Am. Chem. Soc. 1989, 111, 6877.
(46) Dix, E. J.; Herman, M. S.; Goodman, J. L. J. Am. Chem. Soc. 1993,
115, 10424. Storer, J. W.; Houk, K. N. Ibid. 1993, 115, 10426.
Experimental probes of these ideas were carried out by the
Hammett-type substituent variations that are summarized in
Table 1. For the substituted (benzoyloxy)carbenes PhCOOCAr
(27a-c), electron-donating groups modestly accelerate the acyl

12594 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Moss et al.
Table 3. Atomic Natural Charges (NBO Analysis, HF/6-31G*//
6-31G* Level) on Essential Atoms in 27a-c (Z Conformers),
27aTS-cTS, 28a-c (E Conformers), and 28aTS-cTS
species
substituent
q (C1)
q (O1)
q (C2)
q (O2)
27a
27b
27c
27aTS
27bTS
27cTS
28a
28b
28c
H
0.383
0.383
0.381
0.500
0.500
0.497
0.336
0.339
0.346
0.478
0.474
0.475
-0.687
-0.687
-0.686
-0.614
-0.616
-0.616
-0.660
-0.660
-0.659
-0.621
-0.622
-0.620
0.993
0.993
0.995
0.833
0.835
0.837
0.989
0.988
0.988
0.830
0.834
0.835
-0.628
-0.630
-0.638
-0.748
-0.747
-0.751
-0.632
-0.628
-0.624
-0.740
-0.732
-0.727
CH3
OCH3
H
CH3
OCH3
H
Cl
CF3
H
Figure 2. Perspective drawing of transition state 28aTS; see structure
for bond lengths and angles. Note that the perspective drawing is in
mirror image orientation to 28aTS.
28aTS
28bTS
28cTS
Cl
CF3
shift (F ≈ -1), which seems consistent with an acyl anion-like
pathway (Figure 1a). With the arylacetoxycarbenes ArCOAc
(28a-c), electron-withdrawing groups on the aryl moiety bonded
to the migration terminus mildly accelerate the acetyl shift.
These results, too, are in accord with an acyl anion-like transition
state. Alternatively, it is known that electron-withdrawing
groups destabilize carbenes and enhance their reactivity.47
Substituent effects in carbenes 27 and 28 support the acyl
carbanion-like character of the 1,2-acyl migration transition state,
but the picture that emerges from the computational studies
seems at variance with this conclusion. Extensive searches were
made to identify “rearrangement pathways A and B” (see Figure
1), but only a single transition state could be found for the 1,2-
acyl shift in 27a or in 28a. Specifically, the transition state for
1,2-acyl shift (27aTS), illustrated below, is reached starting from
27a-Z; both 28a-E,Z rearrange via 28aTS to 22.
drawing shown in Figure 2. Transition state 27aTS is virtually
identical to 28aTS in all major features.
A population analysis shows modest negative charge buildup
on the carbonyl group (C2,O2) and charge depletion on the
carbene carbon (C1) and O1 occurring in the transition state
relative to the ground state (Table 3). For example, there are
losses of 0.14 e on C1 and 0.04 e on O1 between 28a and 28aTS
and concomitant gains of 0.16 e on C2 and 0.11 e on O2. The
changes are similar for 27a f 27aTS (∆q(C1) ) -0.12 e;
∆q(O1) ) -0.07 e; ∆q(C2) ) 0.16 e; ∆q(O2) ) 0.12 e).
The modest changes in charge computed between the ground
and the transition states imply that rate effects due to substituents
far away from the reaction center are likely to be small. Indeed,
as shown in Table 1, substituting a p-Cl or p-CF3 group on 28a
essentially doubles the rate of the 1,2-acyl shift. The computed
activation energies (MP2/6-31G*//HF/6-31G*, not corrected for
zero-point and thermal energies) are 8.24 kcal/mol for 28a f
28aTS, 8.28 kcal/mol for 28b f 28bTS and 8.08 kcal/mol for
28c f 28cT. The calculations thus fail to pick up the rate
increase induced by the p-Cl substituent.
The computed activation energies are 6.4 kcal/mol for 27a,
6.7 kcal/mol for 27b, and 3.9 kcal/mol for 27c. The experi-
mental rate data (Table 1) show an approximately 50% increase
in acyl shift rate going from 27a to 27b and roughly a doubling
of the rate going from 27a to 27c. Thus, the calculations again
fail to properly reflect the influence of the weak substituent (Me)
and perhaps overestimate the rate-enhancing effect of the OMe
group. It appears that reliable prediction of relatively small
energetic (rate) effects in large molecular systems like 27 and
28 may be slightly beyond the capability of the ab initio methods
presently applicable to the problem.
We are troubled, but unable to explain, the apparent discrep-
ancy between the experimental substituent effects, which seem
more in keeping with expectations for an acyl anion-like
rearrangement pathway, and the calculated transition states (and
associated atomic charges), which appear more consonant with
a carbanion-like attack on the carbonyl group.
Competitive 1,2-H Shift. Due to the strong electron donation
by the methoxy substituent that mitigates the electrophilicity
of the “vacant” carbene 2p orbital or LUMO (cf., 1), typical
1,2-rearrangements involving the R group of 1 and the carbenic
center are suppressed. For example, at ambient temperatures,
cyclopropylmethoxycarbene does not offer ring expansion via
a 1,2-C shift (kc < 3 × 103 s-1),48 and neither neopentyl-
methoxycarbene nor (phenoxymethyl)methoxycarbene afford
alkenes by 1,2-H shifts (k1,2-H < 104 s-1).10 However, because
the acetoxy substituent is a considerably poorer electron donor
than methoxy (cf., 3), we may expect the restoration of other
The calculated geometry for transition state 28aTS seems
more in keeping with a carbanion-like attack of the carbene s
electron pair on the carbonyl LUMO (Figure 1b). The breaking
C2-O1 bond has lengthened from 1.37 Å in the ground state of
the carbene to 1.53 Å in the transition state, and the carbonyl
oxygen is almost perpendicular to the central C1-O1-C2 unit
as evidenced by dihedral angle O2C2O1C1 ) 104.5°. The
dissociating C2-O1 bond is almost orthogonal to the carbene p
orbital; it is the carbonyl group which appears to eclipse the
carbene LUMO in 28aTS. These geometric features resemble
our expectations for the carbanion attack pathway (Figure 1b).
In the acyl anion-like pathway (Figure 1a), the dissociating C2-
O1 bond should eclipse the carbene LUMO. Furthermore, the
carbene-acyl carbon distance (C1C2) has decreased from 2.34
Å in 28a-E (2.33 Å in 28a-Z) to 1.71 Å in 28aTS. The overall
structure of 28aTS is well-represented in the perspective
(47) For example, the additions of ArCCl to various alkenes are
accelerated by electron-withdrawing groups, with F ≈ 1.4-1.6: Moss, R.
A.; Perez, L. A.; Turro, N. J.; Gould, I. R.; Hacker, N. P. Tetrahedron Lett.
1983, 24, 685.
(48) Moss, R. A.; Jang, E. G.; Fan, H.; Wlostowski, M.; Krogh-Jespersen,
K. J. Phys. Org. Chem. 1992, 5, 104.

Reactions and ReactiVity of Acyloxycarbenes
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12595
Table 4. Absolute (and Relative) Rate Constants for Carbene
Phenylacetoxycarbene, in keeping with its oxacarbene iden-
tity, more closely resembles PhCOMe than PhCCl. The
acetoxycarbene is a predominantly nucleophilic ambiphile with
a reactivity toward alkenes that parallels PhCOMe. However,
it is clear from the absolute rate constants that PhCOAc is more
reactive than PhCOMe, particularly toward the alkylethylenes.
Accordingly, PhCOAc is a somewhat more “central” (less
nucleophilic) ambiphile than PhCOMe. The greater intermo-
lecular reactivity and decreased nucleophilicity of PhCOAc,
relative to PhCOMe, are in keeping with the simple resonance
arguments offered above and summarized in structures 1 and
3.
The experimentally expressed ambiphilicity of PhCOAc is
also in harmony with expectations based on the differential
orbital energies calculated for PhCOAc-alkene frontier mo-
lecular orbital interactions.1,7 Thus, for PhCOAc additions to
trimethylethylene, isobutene, or trans-butene the (p-π) orbital
interactions are dominant, whereas with methyl acrylate or the
acrylonitriles, the (σ-π*) orbital interactions dominate.53 The
“crossover” as the substrates change from electron rich to
electron poor is characteristic of ambiphilic carbenes.1,7
Finally, the carbene selectivity index mcxy that can be
calculated8 for PhCOAc from the σ1 and σ+R constants of its
substituents9,13,21 is 0.76, similar to that of PhCCl (0.71).8
However, as we have seen, the alkene selectivity of PhCOAc
more closely resembles that of PhCOMe (mcxy ) 1.34)6 than
that of PhCCl. This agrees with our earlier observation:7 the
mcxy index, which is calibrated with the selectivities of “elec-
trophilic” carbenes, may be useful in guiding the search for
ambiphilic carbenes but cannot be expected to quantitatively
correlate their selectivities. For this purpose, the frontier
molecular orbital approach1,7 is much more effective.
Addition Reactionsa
10-6 kabs, M-1 s-1 (krel)
alkene
PhCOAcb
PhCOMec
PhCCld
Me2CdCHMe
Me2CdCH2
0.99 (4.3)
1.45 (6.3)
0.23 (1.00)
1.20 (5.2)
3.54 (15)
51. (220)
0.013 (3.4)
0.040 (10.6)
0.0038 (1.00)
0.66 (172)
1.7 (445)
130. (16.0)
40. (5.0)
8.1 (1.00)
5.1 (0.50)
5.4 (0.55)
210. (26)f
MeCHdCHMee
CH2dCHCOOMe
CH2dCHCN
CH2dCClCN
34. (8950)f
a At 25 °C. Relative reactivities refer to trans-butene. b This work
and ref 21. c Data from refs 6, 7, and 50. d Data from refs 7 and 51.
e trans-Butene is assigned krel ) 1.00. f Calculated from the kabs values
in this table.
1,2-shifts competitive with 1,2-acyl shifts in appropriate alkyl-
acyloxycarbenes.
(Phenoxymethyl)acetoxycarbene (30) provides an apposite
case. As described above, it undergoes a 1,2-H shift to alkene
29 with kH ) 4.1 × 106 s-1 and no evidence of a competitive
acetyl shift to a dione. In contrast, the strongly donating (σR+
< -0.5) methoxy and trifluoroethoxy substituents “shut off”
1,2-H shifts at ambient temperature (kH < 104 s-1), although
thermal activation at 95 °C can partly restore these reactions.10
Thus, the acetoxy substituent (σ+R ) -0.26)13 permits a highly
competitive 1,2-H shift at 25 °C, behavior closely resembling
that of the chloro substituent (σ+R ) -0.21).13
The 1,2-acetyl shift does not successfully compete with the
1,2-H shift in PhOCH2COAc (30), so that here kacetyl must be
<105 s-1. We note, however, that the phenoxymethyl substitu-
ent is particularly prone to H shifts because the phenoxy unit
augments kH by stabilizing the partial positive charge that
develops at the migration origin as the hydride shift proceeds.26
With other examples of RCOAc, the acetyl shift can compete
more effectively with the H shift.49
Conclusions
Intermolecular Addition. From the absolute rate constants
determined for the additions of PhCOAc to acrylonitrile,
chloroacrylonitrile, and methyl acrylate, together with the
relative reactivities for trimethylethylene, isobutene, and trans-
butene (Table 2), we derived absolute rate constants for additions
of PhCOAc to the latter three alkenes (see above). The kabs
values are collected in Table 4, together with comparable data
for PhCOMe6,50 and PhCCl.51 For convenience, the data in
Table 4 are also presented as relative rate constants, normalized
to trans-butene.
Table 4 indicates that PhCCl can behave as an ambiphile,
displaying latent nucleophilic reactivity when challenged with
the very electrophilic alkene, chloroacrylonitrile.50 However,
its pronounced reactivity toward methyl-substituted alkenes such
as isobutene and trimethylethylene, together with its compara-
tively low reactivity toward methyl acrylate and acrylonitrile,
indicates that the expressed philicity of PhCCl is primarily
electrophilic.52
Arylacyloxycarbenes undergo efficient 1,2-acyl migrations
to afford 1,2-diketones with rate constants in the 105-106 s-1
regime (pentane, 25 °C). The rearrangement of phenyl-
(benzoyloxy)carbene (24) to benzil exhibits Ea ) 8.4 kcal/mol,
log A ) 12.1 s-1, and ∆S* ) -5.0 eu (25 °C). The ab initio
computed activation parameters are Ea ) 5.1 kcal/mol and ∆S*
) -3.3 eu.
Electron-withdrawing substituents in phenylacetoxycarbenes
(28) and electron-donating substituents in phenyl(benzoyloxy)-
carbenes (27) mildly accelerate the 1,2-acyl migrations, in
keeping with an acyl anion-like rearrangement pathway (Figure
1a). However, high-level ab initio calculations of ground and
transition state geometries and charge distributions support a
carbanion-like attack of the lone pair of the carbene on the
carbonyl carbon (Figure 1b).
A 1,2-hydride shift (k ) 4.1 × 106 s-1), rather than a 1,2-
acyl shift, is the kinetically dominant reaction exhibited by
(phenoxymethyl)acetoxycarbene (30).
Phenylmethoxycarbene presents quite a different picture. Its
absolute reactivity toward alkylethylenes is 3-4 orders of
magnitude less than that of PhCCl, but the reactivity increases
steadily as the alkene substrates become more electron deficient.
PhCOMe is therefore an ambiphile of pronounced nucleophilic
character.6,7
Phenylacetoxycarbene (20) undergoes efficient intermolecular
addition to a variety of alkenes, with rate constants ranging from
2.3 × 105 M-1 s-1 (trans-butene) to 5.1 × 107 M-1 s-1 (R-
chloroacrylonitrile). Carbene 20 exhibits ambiphilic selectivity
with a moderate nucleophilic bias, but, in line with the weaker
electron-donating properties of the acetoxy vs the methoxy
group, phenylacetoxycarbene is both more reactive and less
nucleophilic than phenylmethoxycarbene.
(49) Moss, R. A.; Xue, S.; Ma, H. Unpublished work.
(50) Moss, R. A.; Fan, H.; Hadel, L. M.; Shen, S.; Wlostowska, J.;
Wlostowski, M.; Krogh-Jespersen, K. Tetrahedron Lett. 1987, 28, 4779.
(51) Moss, R. A.; Turro, N. J. In Kinetics and Spectroscopy of Carbenes
and Biradicals; Platz, M. S., Ed.; Plenum Press: New York, 1990; pp 213ff
and references therein.
(53) The HOMO (σ) and LUMO (p) orbital energies of PhCOAc are
calculated at -10.13 and 1.61 eV, respectively (see above). The HOMO
(π) and LUMO (π*) orbital energies of the alkenes can be found in ref 1
and 8. The π and π* energies of CH2dCClCN are -10.98 and -0.34 eV,
respectively.50
(52) Moss, R. A.; Lawrynowicz, W.; Hadel, L. M. Tetrahedron Lett.
1986, 27, 4125.

12596 J. Am. Chem. Soc., Vol. 118, No. 50, 1996
Moss et al.
3-Acetoxy-3-(phenoxymethyl)diazirine (13). Similarly, 50 mL of
a DMF solution of (phenoxymethyl)bromodiazirine26 (from 5.0 g, 0.027
mol of (phenoxymethyl)amidine hydrochloride26) was combined with
50 mL of DMF containing 15 g (0.050 mL) of TBA acetate and stirred
under air at 21 °C for 10 min. The usual workup, followed by
chromatography, gave 42% of 13. UV (pentane): λmax 328, 340 (sh)
nm. The NMR is described in ref 21, note 27.
Experimental Section
General. NMR spectra were recorded with a Varian VXR-200
spectrometer; chemical shifts for H and 13C are reported in δ units
1
relative to TMS. For 19F NMR, the chemical shifts are reported in δ
units relative to CFCl3. UV spectra were determined with a Hewlett-
Packard Model 8451A diode array spectrometer. GC-MS spectra were
recorded on an HP 5971 MSD instrument interfaced with an HP 5890
II GC spectrometer (column HP-1), and the data are reported as m/e
(relative intensity). Capillary GC analyses employed a Varian Model
3700 flame ionization instrument, fitted with a Cp-sil 5 CB or HP-1
capillary column, and connected to a Varian Model 4270 integrator.
Preparative GC employed a Varian Model 90-P unit with a 4 ft × 0.25
in. 12% SF-96 Teflon column on 60/70 Anakrom ABS. Photolyses of
diazirines were carried out with a Corning 3-94 uranium glass filter (λ
> 320 nm) using a focused Osram 200 W Xe mercury lamp.
Isobutene and trans-butene were obtained from Matheson and used
as received. All other alkenes were purchased from Aldrich Chemical
Co. and purified before use. Benzamidine hydrochloride was obtained
from Aldrich. The preparation and properties of p-chloro- and
(p-trifluoromethyl)benzamidine hydrochlorides have been described.29
All bromodiazirines were prepared by hypobromite oxidation
(Graham’s reaction22) of the appropriate amidine. These procedures
have been described for phenylbromodiazirine (8) and its p-Cl and
p-CF3 derivatives.29
3-(Benzoyloxy)-3-phenyldiazirine (14). Potassium benzoate (1.36
g, 8.5 mmol) was dissolved in 60 mL of anhydrous DMF with the aid
of 1.4 g (5.3 mmol) of 18-crown-6. This solution was combined with
40 mL of DMF containing phenylbromodiazirine22,29 prepared from
2.5 g (1.6 mmol) of benzamidine hydrochloride. Then 0.30 g (3.1
mmol) of potassium acetate was added, and the mixture was stirred
under air at 25 °C for 8 h. The usual workup afforded a pentane
concentrate that contained 32% of acetoxydiazirine 9, 28% of the
desired (benzoyloxy)diazirine 14, and 5% of phenyldiazirine 10.24
Diazirine 14 was purified by chromatography on silica gel (4:1
pentane-CH2Cl2). UV (pentane): λmax 368, 380 (sh) nm. 1H NMR
(CDCl3): 6.96-7.01 (m, 2H, Ph), 7.34-7.38 (m, 3H, Ph), 7.44-7.52
(t, 2H, J ) 7.9 Hz, PhCOO), 7.60-7.68 (t, J ) 7.0 Hz, 1 H, PhCOO),
8.05-8.10 (d, J ) 7.9 Hz, 2H, PhCOO). 13C NMR (CDCl3): 55.41,
124.87, 127.99, 128.65, 128.75, 129.08, 130.22, 133.73, 134.29, 164.02.
3-(p-Methyl(benzoyloxy))-3-phenyldiazirine (16). Phenylbromo-
diazirine (8), prepared from 3.0 g (19 mmol) of benzamidine hydro-
chloride,22,29 in 50 mL of DMF was added to 50 mL of a DMF solution
of 7.9 g (21 mmol) of TBA p-methylbenzoate. Then 0.50 g (5.6 mmol)
of KOAc was added, and the mixture was stirred under air for 5 h at
25 °C. The usual workup and chromatography gave 43% of diazirine
16, accompanied by ∼25% of phenylacetoxydiazirine (9).55 UV
(pentane): λmax 366, 378 nm. NMR (CDCl3): 2.45 (s, 3H, Me), 6.97-
7.03 (m, 2H, Ph), 7.30 (d, J ) 8 Hz, 2H, ArCOO), 7.35-7.40 (m, 3H,
Ph), 7.99 (d, J ) 8 Hz, 2H, ArCOO).
3-(p-Methoxy(benzoyloxy))-3-phenyldiazirine (17). Similarly, from
phenylbromodiazirine prepared from 3.0 g (19 mmol) of benzamidine
hydrochloride, 11.8 g (30 mmol) of TBA p-methoxybenzoate and 0.50
g (5.6 mmol) of KOAc stirred in 100 mL of DMF for 2 h at 25 °C, we
obtained 80% of diazirine 17, accompanied by ∼7% of diazirine 9.
The desired 17 was purified by chromatography on silica gel (3.5:1
pentane-CH2Cl2). UV (pentane): λmax 368, 380 (sh) nm. NMR
(CDCl3): 3.89 (s, 3H, MeO), 6.94-7.03 (m, 4H, ArCOO + Ph), 7.36-
7.39 (m, 3H, Ph), 8.05 (d, J ) 8.8 Hz, 2H, ArCOO).
The tetrabutylammonium (TBA) salts of p-methylbenzoate, p-
methoxybenzoate, and trimethylacetate were prepared from the corre-
sponding acid by the “ion pair extraction” method.54 A solution of
0.1 mol of the carboxylic acid in 30 mL of water was added to 26 g
(0.10 mol) of 40% aqueous TBAOH (Aldrich). Then 150 mL of CH2-
Cl2 was added, and after shaking, the organic layer was separated and
retained. The aqueous phase was extracted with 3 × 50 mL of CH2-
Cl2, and the combined CH2Cl2 solutions were dried (MgSO4), filtered,
and stripped at 25 °C. The TBA salt thus obtained was dissolved in
50 mL of MeCN, and the solvent was stripped on the rotary evaporator.
This drying process was repeated, and the TBA salt was dissolved in
80 mL of anhydrous DMF. The DMF volume was reduced to about
50 mL by rotary evaporation. The DMF TBA salt solution was retained
for the syntheses described below.
Elemental analyses were performed by Quantitative Technologies,
Inc., Whitehouse, NJ.
3-Acetoxy-3-phenyldiazirine (9). A solution of 10 g (0.033 mol)
of TBA acetate in 50 mL of DMF was combined with a solution of
phenylbromodiazirine (8) prepared from 6.0 g (0.038 mol) of benza-
midine hydrochloride22,29 and contained in 50 mL of dry DMF. The
combined DMF solution was stirred under air at 25-30 °C for 20 min
and then poured into a large separatory funnel that contained 1000 mL
of ice cold brine and 150 mL of pentane. The pentane extract was
separated, washed with 3 × 600 mL of water, and then dried over
CaCl2. The pentane extract was reduced to ∼2 mL, and the desired
diazirine 9 was purified from accompanying phenyldiazirine (10) by
chromatography over silica gel, using pentane-CH2Cl2 (4:1) as the
eluent. The yield55 of 3-acetoxy-3-phenyldiazirine was variable (∼45%)
based on the bromodiazirine consumed and the reaction time. UV (pen-
tane): 345 (sh), 364, 378 nm.56 NMR (CDCl3): 2.18 (s, 3H, Me),
6.90-7.00 (m, 2H, Ph), 7.36-7.40 (m, 3H, Ph).
3-Phenyl-3-(trimethylacetoxy)diazirine (12). In a similar manner,
50 mL of a DMF solution of phenylbromodiazirine (from 3 g, 0.019
mol, of benzamidine hydrochloride) was combined with 50 mL of DMF
containing 12 g (0.035 mol) of TBA trimethylacetate and stirred at 25
°C for 1 h. A workup as for 9, followed by analogous chromatography,
afforded a pentane-CH2Cl2 solution of 12 in about a 60% yield, based
on consumed bromodiazirine. UV (pentane): λmax 366, 378 nm. NMR
(CDCl3): 1.27 (s, 9H, CMe3), 6.90-6.96 (m, 2H, Ph), 7.35-7.39 (m,
3H, Ph).
3-Acetoxy-3-(p-chlorophenyl)diazirine (18). (p-Chlorophenyl)-
bromodiazirine,29 prepared from 3.0 g (16 mmol) of p-chlorobenza-
midine hydrochloride29 in 50 mL of DMF was added to 6.2 g (21 mmol)
of TBA acetate in 50 mL of DMF. The mixture was stired for 10 min
at 25 °C and then worked up as usual. Chromatography on silica gel
(1:1 pentane-CH2Cl2) afforded 69% of diazirine 18 as well as 10% of
(p-chlorophenyl)diazirine. UV: 347 (sh), 366 (λmax), 375, 386 nm.
NMR (CDCl3): 1.98 (s, 3H, MeCOO), 6.78 (d, J ) 8.6 Hz, 2H, Ar),
7.23 (d, J ) 8.6 Hz, 2H, Ar).
3-Acetoxy-3-(p-(trifluoromethyl)phenyl)diazirine (19). p-(Tri-
fluoromethyl)phenylbromodiazirine,29 prepared from 3 g (13 mmol)
of p-trifluoromethylbenzamidine hydrochloride29 and 6.2 g (21 mmol)
of TBA acetate in 100 mL of DMF were stirred for 5 min at 25 °C and
then worked up as usual. The desired diazirine (19) was formed in
54% yield and purified by silica gel chromatography (1:1 pentane-
CH2Cl2). p-(Trifluoromethyl)phenyldiazirine was also formed in 2%
yield. UV (pentane): λmax 358, 370 nm. NMR (CDCl3): 2.17 (s, 3H,
MeCOO), 7.04 (d, J ) 8.3 Hz, 2H, Ar), 7.57 (d, J ) 8.3 Hz, Ar).
Diones. Photolyses of the various diazirines (except for 13) afforded
dione products by 1,2-acyl migration. General procedure: A pentane
solution (30 mL) of the diazirine, A ) 0.4 at λmax, was photolyzed for
1.5 - 2 h with λ > 320 nm at 25 °C. The UV spectrum showed that
no diazirine remained. Removal of pentane gave the dione, which was
identified either by comparison with literature data or an authentic
sample or characterized by standard methods.
(54) Brandstrom, A.; Berntsson, P.; Carlsson, S.; Djurhuus, A.; Gustavii,
K.; Junggren, U.; Lamm, B.; Samuelsson, B. Acta Chem. Scand. 1969, 23,
2202.
(55) All diazirine yields were determined by NMR and are based on
consumed bromodiazirine.
(56) In ref 21, note 18, λmax of 9 is erroneously given as 336 instead of
364 nm.
1-Phenyl-1,2-propanedione (22) was obtained in >90% yield by
photolysis of diazirine 9.21
1-Phenyl-3,3-dimethyl-1,2-propanedione (25) was obtained in >90%
yield by photolysis of diazirine 12. NMR (CDCl3): 1.30 (s, 9H, CMe3),
7.49 (t, J ) 7.3 Hz, 2H, Ph), 7.64 (t, J ) 7.3 Hz, 1H, Ph), 7.82 (t, J

Reactions and ReactiVity of Acyloxycarbenes
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12597
) 7.3 Hz, 2H, Ph). Anal. Calcd for C, 75.7; H, 7.42. Found: C,
75.5; H, 7.06.
Benzil (26) was obtained in >90% yield by photolysis of diazirine
14. It was identified by comparison with an authentic sample.
Thermolysis of 14 in pentane (60 °C, 30 h, sealed tube) gave >98%
of benzil.
(CDCl3): 0.80 (s, 3H, Me), 0.90 (d, J ) 6.7 Hz, 1H, CH), 1.33 (s, 3H,
Me), 1.34 (d, J ) 6.7 Hz, 1H, CH), 1.96 (s, 3H, OOCMe), 7.23-7.36
(m, 3H, Ph), 7.41-7.46 (m, 2H, Ph). Anal. Calcd for C13H16O2: C,
76.4; H, 7.90. Found: C, 76.4; H, 7.66.
1-Acetoxy-1-phenyl-trans-2,3-dimethylcyclopropane (31c). Pho-
tolysis of 9 in trans-butene gave 60% (NMR yield) of 31c. GC-MS
m/e: M+ 204. NMR (CDCl3): 0.79 (d, J ) 6.5 Hz, 3H, Me), 1.06 (q,
J ) 6.5 Hz, 1H, CH), 1.21 (d, J ) 6.2 Hz, 1H, Me), 1.35 (q, J ) 6.2
Hz, 1H, CH), 2.00 (s, 3H, OOCMe), 7.25-7.37 (m, 3H, Ph), 7.40-
7.45 (m, 2H, Ph). Anal. Calcd for C13H16O2: C, 76.4, H, 7.90.
Found: C, 76.0, H, 7.71.
p-Methylbenzil57 (lit. mp 29-30 °C) was obtained in >92% yield
by photolysis of diazirine 16. GC-MS m/e: M+ 224. NMR (CDCl3):
2.48 (s, 3H, Me), 7.36 (d, J ) 8.3 Hz, 2H, Ar), 7.55 (t, J ) 7.1 Hz,
2H, Ph), 7.70 (t, J ) 7.1 Hz, 1H, Ph), 7.92 (d, J ) 8.3 Hz, 2H, Ar),
8.02 (d, J ) 7.1 Hz, 2H, Ph).
p-Methoxybenzil57 (lit. mp 64 °C) was obtained in >93% yield by
photolysis of diazirine 17. GC-MS m/e: M+ 240. NMR (CDCl3):
3.89 (s, 3H, Me), 6.98 (d, J ) 9 Hz, 2H, Ar), 7.50 (t, J ) 7.3 Hz, 2H,
Ph). 7.65 (t, J ) 7.3 Hz, 1H, Ph), 7.92-7.99 (m, 4H, 2Ar + 2Ph).
1-(p-Chlorophenyl)-1,2-propanedione58 (lit.57 mp 72-73 °C) was
obtained in >90% yield by photolysis of diazirine 18. GC-MS m/e:
M+ 182, 184. NMR (CDCl3): 2.51 (s, 3H, Me), 7.45 (d, J ) 8.6 Hz,
2H, Ar), 7.96 (d, J ) 8.6 Hz, 2H, Ar).
1-Acetoxy-1-phenyl-2-(carbomethoxy)cyclopropane (31d). Pho-
tolysis of 9 in the presence of methyl acrylate afforded >90% of 31d
as a 2.7:1 mixture of syn and anti isomers. NMR (CDCl3): major
isomer, 1.63-1.72 (m, 2H, CH2), 2.04 (s, 3H, OOCMe), 2.42 (dd, J )
7.5, 7.7 Hz, 1 H, HCCOOMe), 3.74 (s, 3H, COOMe), 7.2-7.6 (m,
5H, Ph); minor isomer, 1.63-1.72 (m, 2H, CH2), 1.96 (s, 3H, OOCMe),
2.17 (“t”, J ) 7.5 Hz, HCCOOMe), 3.46 (s, 3H, COOMe), 7.2-7.6
(m, 5H, Ph). On the basis of the shielding effect of Ph relative to
OAc, the major isomer is provisionally assigned the syn-OAc/COOMe
configuration. Exact mass: calcd for C13H14O4 234.0891, found
234.0892.
1-(p-(Trifluoromethyl)phenyl)-1,2-propanedione59 (mp 42-44 °C)
was obtained in >90% yield by photolysis of diazirine 19. GC-MS
m/e: M+ 216. 1H NMR (CDCl3): 2.57 (s, 3H, Me), 7.76 (d, J ) 8.1
Hz, 2H, Ar), 8.14 (d, J ) 8.1 Hz, 2H, Ar). 19F NMR (CDCl3): -63.78.
The photolysis of diazirine 13 afforded >90% of alkene 29, which
has been described previously.21
1-Acetoxy-1-phenyl-2-cyanocyclopropane (31e). Photolysis of
diazirine 9 in acetonitrile afforded ∼90% of 31e as a 1.2:1 mixture of
syn and anti isomers. NMR (CDCl3, both isomers): 1.82-2.22 (m,
2H, CH2), 2.03 (s, 3H, OOCMe, minor isomer), 2.16 (s, 3H, OOCMe,
major isomer), 2.62-2.78 (m, HCCN, both isomers), 7.3-7.7 (m, 5H,
Ph). Exact mass: calcd for C12H12NO2 202.0868, found 202.0868.
Cyclopropanes 31. General Procedure. A 30 mL pentane solution
of phenylacetoxydiazirine 9 (A366 ) 10, ∼1.6 mmol) was added to ∼100
mmol of alkene (condensed at -20 °C in the cases of isobutene and
trans-butene). The resulting solution was photolyzed (λ > 320 nm)
for 2 h at 25 °C (using a screw-top Carius tube for gaseous alkenes).
Solvent and excess alkene were removed by preparative GC on a 4 ft
12% SF-96 on 60/70 Anakrom ABS column at 150 °C with 60 mL/
min of He carrier gas.
1-Acetoxy-1-phenyl-2,2,3-trimethylcyclopropane (31a). Photolysis
of 9 in the presence of 2-methyl-2-butene gave 81% of a 11:1 mixture
of syn and anti 31a; isomer identities are unassigned.60 GC-MS m/e:
M+ 218 (each isomer). NMR (CDCl3): major isomer, 0.95 (s, 3H,
Me), 1.01 (d, J ) 6 Hz, 3H, Me), 1.29 (s, 3H, Me), 0.96-1.13 (m,
1H, CH), 1.90 (s, 3H, OOCMe), 7.25-7.37 (m 3H, Ph), 7.46-7.51
(m, 2H, Ph); minor isomer, 0.75 (s, 3H, Me), 1.07 (d, J ≈ 6 Hz, 3H,
Me), 1.16 (s, 3H, Me), 1.98 (s, 3H, Me) (Ph protons as for major
isomer). Anal. Calcd for C14H18O2: C, 77.0; H, 8.32. Found: C,
76.8; H, 8.06.
1-Acetoxy-1-phenyl-2-chloro-2-cyanocyclopropane (31f). Pho-
tolysis of diazirine 9 in R-chloroacrylonitrile afforded ∼90% of 31f as
a 1.3:1 mixture of isomers. NMR (CDCl3): isomer a, 2.01 (d, J ) 9.1
Hz, 1H, CH), 2.11 (s, 3H, OOCMe), 2.70 (d, J ) 9.1 Hz, 1H, CH),
7.3-7.6 (m, 5H, Ph); isomer b, 2.10 (s, 3H, OOCMe), 2.32, 2.36, 2.39,
2.44 (AB, J ) 8 Hz, 2H, CH2), 7.3-7.6 (m, 5H, Ph). Exact mass:
calcd for C12H11ClNO2 (M+ + H+) 236.0478, found 236.0480.
Kinetics. Relative reactivities were determined by standard meth-
ods34 as described in the results; experimental relative reactivities of
PhCOAc toward various alkenes appear in Table 2.
Absolute rate constants for the various acyloxycarbenes were
determined on our LFP installation which has been described previ-
ously.6 The installation has been upgraded by the addition of a
Tektronix TDS 520A programmable digitizer which generates a 512-
point trace with a minimum resolution of 0.5 ns/point. Data were
analyzed with the program Igor Pro 2.0 (WaveMatrices, Inc.). Kinetic
methodology and results are extensively described above.
1-Acetoxy-1-phenyl-2,2-dimethylcyclopropane (31b). Photolysis
of 9 in isobutene led to 86% of 31b. GC-MS m/e: M+ 204. NMR
(57) Armesto, D.; Horspool, W. M.; Ortiz, M. J.; Perez-Ossorio, R.
Synthesis 1988, 799.
(58) Ahmad, S.; Iqbal, J. J. Chem. Soc., Chem. Commun. 1987, 692.
(59) Strom, E. T. J. Am. Chem. Soc. 1966, 88, 2065.
(60) In view of the higher field appearance of the OOCMe singlet and
lower field appearance of the upfield cyclopropyl Me singlet of the major
isomer, this isomer is likely to be of the syn-OAc configuration. For NMR
shielding effects in phenylcyclopropanes, see: Closs, G. L.; Moss, R. A. J.
Am. Chem. Soc. 1964, 86, 4042.
Acknowledgment. Dedicated to Professor Donald B. Den-
ney on the occasion of his retirement. We are grateful to the
National Science Foundation for financial support. We thank
Professor Ronald Sauers for help with Figure 2.
JA9626244